function file_size(sequence file_name)
object d = dir(file_name)
    if atom(d) or length(d)!=1 then return -1 end if
    return d[1][D_SIZE]
end function

procedure test(sequence file_name)
integer size = file_size(file_name)
    if size<0 then
        printf(1,"%s file does not exist.\n",{file_name})
    else
        printf(1,"%s size is %d.\n",{file_name,size})
    end if
end procedure

test("input.txt") -- in the current working directory
test("/input.txt") -- in the file system root